Optocouplers provide a means of isolating two electrical circuits that need to communicate signals with each other but can't be electrically tied to one another. One reason for the electrical isolation is that electrical noise generated in one circuit, for example an electric motor switching circuit, may upset operations in another circuit, for example a microcontroller that controls the motor. Another reason is to eliminate an electrical hazard for humans by electrically isolating a dangerous high voltage circuit.
Optocouplers are able to send signals from one circuit to a second circuit using light instead of wires by incorporating a Light Emitting Diode (LED) to generate a light signal from an electrical signal and a light detector to receive the light signal and convert it back into an electrical signal. The typical LED uses a GaAsP based material and the typical photo detector uses a silicon based PN junction diode. These two dissimilar materials, GaAsP and silicon, require a hybrid package construction. Also, it is not practical to make transistors in the GaAsP material. Thus, optocouplers today are largely confined to providing just the basic signal isolation function and do not include surrounding system circuitry.
The most desirable implementation of an optocoupler from a cost and circuit density point of view is to have the optocoupler function imbedded into a silicon system chip. In fact, in power systems the trend has been to imbed digital logic into power chips to make what is termed Power Management Integrated Circuits (PMCs). However, the optocoupler function remains external to PMICs. The limiting factor has been that silicon based LEDs have poor light emission efficiency. However, there is enough light emission efficiency from a silicon PN junction to make practical, all silicon optocouplers if a high gain amplifier is connected to the output of the photo detector. With silicon on insulator (SOI), isolation of the LED and photo detector is achieved using transparent insulating materials. Furthermore, different system circuits can be made in the same SOI material as the optocoupler.
There are at least two ways in which a silicon PN junction can emit light. In an avalanche or reverse breakdown mode, silicon emits visible light that is yellow in color to the naked eye. In a forward mode, silicon emits infra (IR) light. U.S. Pat. No. 6,365,951 discloses methods for making silicon based LEDs. Recent reports in literature show that when properly constructed, the light efficiency of a silicon PN junction operated in the forward mode is much more efficient than previously thought. Furthermore, silicon PN junction diodes can detect both the visible light emitted the reverse breakdown mode and the IR light emitted in the forward mode. However, the silicon PN junction is not as efficient at detecting IR light as it is for visible light.
In U.S. Pat. No. 5,438,210 an all silicon opto-coupler was proposed using silicon on insulator (SOI) material. The present disclosure builds on the concepts of this patent and provides more detail on constructing an SOI based, all silicon optocoupler that can be made using standard IC processing methods.